Erosion has created a 300-foot-deep hole in the concrete spillway of Oroville Dam and state officials say it will continue grow.
State engineers on Wednesday cautiously released water from Lake Oroville's damaged spillway as the reservoir level climbed amid a soaking of rain.
Situated in the western foothills of the Sierra, Lake Oroville is the second-largest manmade reservoir in California after Shasta....
Mtown's posted pdf file includes this comment from said consultants at page 7:
"Chute Spillway [Gated Spillway]. The chute spillway [Gated Spillway] has operated many times since its completion in 1968. Although the floor of the spillway chute [Gated Spillway] in this section has experienced a good deal of spalling and cracking, there has been no significant damage. The cracks and spalls have been repaired several times."
I.e. the floor of the spillway has been accumulating damage (spalls and cracks) every time it is run, likely from cavitation. "No significant damage" is a phrase that just sets off alarm bells for me, it is just too similar to the language used by NASA prior to (and in some cases after) the Columbia shuttle disaster. "It hasn't failed catastrophically yet"
Underfloor leakage (side drains run full when the spillway gates are opened) and subsidence is also noted, but the source of that water is not clear, but is likely moving through cracks and leaking expansion joint seals along the floor of the spillway. I.e. cavitation damage leads to leakage, leads to subsidence and further slab breakup, more cavitation and erosion, and a literal cascading disaster develops...
I read that as simply saying the concrete had not aged very well. When I read it, I had believed it said the remaining floor simply wasn't in good condition. Another part indicated the concrete was thin above each drain pipe and in certain other areas. It read like poor initial construction was the root cause moreso than anything else. It also seems the chute is going to be re-built, but done better this time.
There is a Viewpoint article in the latest Engineering News Record that is very misleading and, in my opinion should not have been printed. Take a look at it and the headlines. How to Fix Oroville Dam. makes it look simple when it really isn't. The reference is not a complete copy of the article. See the actual magazine to read it all.
"As an experienced civil engineer, I would like to make one point very clear: Oroville Dam was never in danger of failing."
Isn't the emergency spillway part of the dam? When the wall was overflowing and it appeared earth around the wall was washing away, was it not conceivable that the wall could have failed? No-one could see what was happening exactly or what damage was being done while it was overflowing. Should the authorities have erred on the side of caution or not?
"was it not conceivable that the wall could have failed?"
The failure concern was only the wall height between the normal spillway and the emergency spillway, which would have released 30 ft of water. The "dam" itself was going to be intact, retaining a few hundred feet of water that's below the normal spillway.
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!
My reason for posting the discussion section from the ENR WEB site was the fellow's off hand cure for the problem, which was printed in the final section of the magazine's article that didn't get carried to the discussion WEB pages. I felt the apparent endorsement of an idea by a national engineering magazine was a mistake. A simplified "cure" or "fix" (calling for a second service spillway at a predetermined location) dismisses all the side issues, all of which played a role in the failure and should be handled with any fix.
There has always been a semantics issue with this event. The media talking about "dam failure" where
the entire Oroville staff understands the dam to be the actual man made gravity dam. Their
hackles go up every time someone sez the dam was about to fail as it wasn't ever about to fail.
Meanwhile, others consider "the dam" to be the entire reservoir containment. The 'reservoir
containment' was indeed threatening to fail due to foot cutting while the emergency spillway was
running - enough of a threat to panic evacuate 150k people.
And, "only the top 30 feet" is a total joke as having 'only' a million acre feet of water rushing
uncontrolled across any part of the reservoir's containment would have cut clear to the bottom exactly
like the main spillway's misguided water did.
Thanks Keith for pointing out what I thought was somewhat obvious.
If that spillway wall did fail, does anyone here actually believe the earth below wouldn't be eroded as that much water washed uncontrolled over that area? There is no possible way anyone can say that if the wall had failed that "only" water the height of the wall would be released.
And even if the earth miraculously wasn't eroded, does anyone here actually believe that only 30' water quickly leaving the reservoir wouldn't be considered a catastrophic failure of the dam? A dam is basically a structure built to hold back water, so I consider the overflows as part of the dam.
oldestguy - yes, he made it out to be simple to fix, when it's not. I really didn't find it a big surprise after his rather dubious claim right off the top.
